According to various image compression standards, especially MPEG ("Motion Pictures Experts Group"), the images are decoded in blocks, or macroblocks, generally of 16.times.16 pixels. The macroblocks can be of various formats. The most commonly used format is the one termed 4:2:0 according to which each macroblock contains four blocks of 8.times.8 luminance pixels of eight bits and two blocks of 8.times.8 chrominance pixels of eight bits.
The images processed are essentially of three types, namely an "intra" type, a "predicted" type and a "bidirectional" type. The person skilled in the art is aware that the macroblocks of an "intra" image do not undergo any motion compensation. In a predicted image, each macroblock can undergo a motion compensation which includes combining the macroblock with another macroblock, the "predictor", fetched from a previously decoded image. Each macroblock of a bidirectional image can undergo a motion compensation which includes combining the macroblock with two other predictor macroblocks, fetched respectively from two previously decoded images. The positions of the predictor macroblocks are determined by motion vectors.
An MPEG decoding/display system, referred to more simply hereafter as an "MPEG decoder", must communicate to a dynamic memory to carry out the decoding and the displaying of the decoded images. Such a memory plays an essential role in the decoding and displaying of these images. In certain modes of operation (freeze frame for example), certain images have to be decoded several times. These multiple decodings require repeated access to the area of the image memory storing the compressed image data awaiting processing. In particular, it is especially important to be able to redecode, without the risk of errors originating from the addressing of the memory, the right compressed data actually corresponding to an image already previously decoded.